1. Field of the Invention
The present invention relates to a read only memory (hereinafter, abbreviated as "ROM" ) such as a mask ROM, or an EPROM.
2. Description of the Related Art
FIG. 1 is a block diagram showing the configuration of a prior art ROM. In the figure, 80 designates the ROM, 83 designates a data storage unit which stores data, 84 designates an address signal line for a read address which is externally given in order to read out data from the data storage unit 83, and 86 designates a data bus through which data read out from the data storage unit 83 are outputted to the outside of the ROM 80.
The procedure of reading out data from the thus configured prior art ROM 80 will be described. At first, a read address is supplied from the outside through the address signal line 84 to the data storage unit 83. Then, the data storage, unit 83 selects data stored at the storage area which is indicated by the read address, and outputs the selected data to the outside of the ROM 80 through the data bus 86, thereby completing the reading of data.
For example, the case where data having the configuration shown in FIG. 2 are stored in the data storage unit 83 will be considered. When address `0` is sent through the address signal line 84, address `0` is given as it is to the data storage unit 83. The data storage unit 83 selects data `A` stored at the storage area corresponding to address `0` and outputs it to the outside of the ROM 80.
As described above, in a prior art ROM, data are stored as they are or without being subjected to a process such as a compression. This produces a problem in that the storage requirement is increased in proportion to an increase of the amount of data.
Further, since, in a prior art ROM, data are stored as they are or without being subjected to a process such as a compression, it is easy to unduly copy data stored in the ROM.